Method for fabricating DRAM cell using a protection layer

ABSTRACT

A DRAM cell is provided, along with a method for fabricating such a DRAM cell. A protection layer pattern is formed to cover a common drain region of first and second access transistors. Storage node holes are then formed to expose each source region of the first and second access transistors, by using an etching insulator that has an etching selectivity with respect to the protection layer. Accordingly, even if there is a misalignment of the storage node holes to thesource regions, the common drain region is not exposed by the misaligned storage node holes because of the presence of the protection layer pattern.

This application relies for priority upon Korean Patent Application No. 99-48926, filed on Nov. 5, 1999, the contents of which are herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor memory device and a method for fabricating such a device. More particularly, the present invention relates to a DRAM cell and a method for fabricating such a DRAM.

Semiconductor memories are considered one of the crucial microelectronics components for mainframe computers, PCs, telecommunications, automotive and consumer electronics, and commercial and military avionics systems. Semiconductor memory devices can be characterized as either volatile random access memory devices (RAMs) or non-volatile memory devices (NVMs). RAMs can further include dynamic RAMs (DRAMs) and static RAMs (SRAMs). As is well known, DRAMs have about four times as high a degree of integration compared to SRAMs. Because of this, DRAMs have been widely used in computer main memories.

DRAMs are composed of a cell array region that has a plurality of memory cell arrays, and a peripheral circuit region that controls and drives the memory cell arrays. Each memory cell typically consists of a cell storage capacitor and an access transistor. Either the source or drain of the access transistor is connected to one terminal of the cell capacitor. The properties of the cell storage capacitor directly affects the characteristics of the DRAM, such as data retention, soft error rate, low voltage performance, or the like. In particular, a higher capacitance of the cell capacitor improves the data retention characteristics and low voltage characteristics, and reduces the soft error rate of the DRAM. Accordingly, in order to realize high density DRAM devices, the cell capacitor is formed to have an acceptable level of capacitance in a given cell.

U.S. Pat. No. 5,597,756 by Fazan et al entitled as “PROCESS FOR FABRICATING A CUP-SHAPED DRAM CAPACITOR USING A MULTI-LAYER PARTIALLY-SACRIFICIAL STACK”, the disclosure of which is incorporated herein by , >reference, discloses a capacitor storage node having an HSG silicon layer on its surface. Also, U.S. Pat. No. 5,907,772 by Iwazaki entitled as “METHOD FOR PRODUCING CYLINDRICAL STORAGE NODE OF STACKED CAPACITOR IN MEMORY CELL”, the disclosure of which is incorporated herein by reference, discloses a planarized interlayer insulating layer formed on a semiconductor substrate. The semiconductor substrate has an access transistor and a cylindrical storage node formed on the planarized interlayer insulating layer, which is electrically connected to a source region of the access transistor.

FIGS. 1 to 5 are cross-sectional views of a conventional semiconductor substrate, at selected stages of a DRAM fabrication process. Referring to FIG. 1, a device isolation region 23 is formed in a predetermined region of a semiconductor substrate 21 to define an active region. A gate oxide layer 25 is formed on the active region. Doped polysilicon and a silicon nitride layers are sequentially formed on the resulting structure. The doped polysilicon and the silicon nitride layers are patterned to form a first and a second gate patterns 30 a and 30 b, intersecting the active region and neighbouring each other. The first gate pattern 30 a comprises a stacked layer of a first polysilicon pattern 27 a and a first silicon nitride layer pattern 29 a. Similarly, the second gate pattern 30 b comprises a stacked layer of a second polysilicon pattern 27 b and a second silicon nitride layer pattern 29 b. The first and second polysilicon patterns 27 a and 27 b respectively serve as a gate electrode of neighbouring access transistors.

A silicon nitride layer is formed on the entire surface of the semiconductor substrate 21 having the first and second gate patterns 30 a and 30 b and is then anisotropically etched to form a side wall spacer 31 on side walls of the first and second gate patterns 30 a and 30 b.

Referring now to FIG. 2, an insulating layer 33, such as a CVD oxide layer, is formed on the resulting structure. Selected portions of the insulating layer 33 are then etched to form pad contact holes that respectively expose the active regions outside of the first and second gate patterns 30 a and 30 b. A contact hole, defined between the first and second gate patterns 30 a and 30 b, is a bit line pad contact hole. Contact holes outside of the bit line pad contact hole are respectively first and second storage node pad contact holes, respectively. A bit line pad 35 d and first and second storage node pads 35 a and 35 b are formed by respectively filling the bit line pad contact hole and the first and second storage node pad contact holes with a conductive material.

Referring now to FIG. 3, a first interlayer insulating layer 37, such as a CVD oxide, is formed on the resulting structure. A selected portion of the first interlayer insulating layer 37 is then etched to form a bit line contact hole (not shown), which exposes the bit line pad 35 d. The bit line contact hole is then filled with a conductive material to form a bit line (not shown). A second interlayer insulating layer 39, an etching stopper layer 41, and a sacrificial insulating layer 43 are then sequentially formed on the first interlayer insulating layer 37 including the bit line. The second interlayer insulating layer 39 comprises a CVD oxide layer planarized by CMP process. The etching stopper layer 41 comprises a material having an etching selectivity with respect to an oxide, such as a silicon nitride layer. The sacrificial insulating layer 43 comprises a CVD oxide.

Referring now to FIG. 4, the sacrificial insulating layer 43, the etching stopper layer 41, the second interlayer insulating layer 39, and the first interlayer insulating layer 37 are sequentially patterned to form first and second storage node holes 45 a and 45 b, respectively exposing the first and second storage node pads 35 a and 35 b. At this time, the first and second storage nodes holes 45 a and 45 b can be misaligned to the storage node pads 35 a and 35 b, thereby exposing the bit line pad 35 d, as shown in FIG. 4.

A conformal conductive layer is deposited on the sacrificial insulating layer 43 and in the holes 45 a and 45 b to form first and second cylindrical storage nodes 47 a and 47 b. As noted above, in the case of a misalignment of the first and second storage node holes 45 a and 45 b, one of the first and second storage nodes 47 a and 47 b, for example, the second storage node 47 b, becomes electrically connected to the bit line pad 35 d.

Referring to FIG. 5, the sacrificial insulating layer 43 is etched to expose outer sidewalls of the first and second cylindrical storage nodes 47 a and 47 b. The etching stopper layer 41 serves as an end-point of the etching and thus the second interlayer insulating layer 39 is not exposed.

As described above, when misalignment occurs during a photolithography process for forming storage node holes, the storage node and the bit line pad may be electrically connected, causing a malfunction of the fabricated DRAMs.

SUMMARY OF THE INVENTION

The present invention was made in view of above-mentioned problems and it is an object of the present invention to provide a method of fabricating a DRAM cell that can prevent bit line pad from being exposed by the storage node hole even when there is misalignment of the storage node hole during a photolithography process.

It is another object of the present invention to provide a method of fabricating a high performance cell capacitor compatible with high density DRAMs.

It is still another object of the present invention to provide a DRAM cell in which the storage node and the bit line pad are electrically separated even when there is misalignment of the storage node hole to the bit line.

In accordance with the present invention, a method is provided for fabricating a cylindrical capacitor. The method includes forming first and second access transistors that share a common drain region over a semiconductor substrate. A first interlayer insulating layer is formed on the semiconductor substrate and the first and second access transistors. A protection layer pattern is provided that completely covers the common drain region on the first interlayer insulating layer. An insulator is formed over the first interlayer insulating layer and the protection layer pattern. The insulator includes a second interlayer insulating layer. The insulator and the first interlayer insulating layer are sequentially patterned to form first and second storage node holes, respectively exposing a first source region of the first access transistor and a second source region of the second access transistor. First and second storage nodes are formed in the first and second storage node holes, respectively.

The forming of the first and the second access transistors may be performed by forming a device isolation layer over a selected portion of the semiconductor substrate to define a primary active region; forming first and second insulated gate patterns crossing over the primary active region to divide the primary active region into first and third active regions; forming sidewall spacers on sidewalls of the first and second gate patterns; and forming the common drain region at the first active region between the first and second gate patterns, and the first and the second source regions at the second and third active regions next to the first and second insulated gate patterns, respectively.

The method for fabricating a DRAM cell may further include forming a bit line pattern between the protection layer pattern and the common drain region.

The forming of the protection layer pattern may be performed by forming the protection layer over the first interlayer insulating layer and over the bit line pattern; forming a photoresist pattern over the protection layer to cover the common drain region and expose the first and second source regions; and anisotropically etching the protection layer using the photoresist pattern as an etching mask to form the protection layer pattern completely covering bit line pattern over the common drain region, while concurrently forrning sidewall spacers on sidewalls of the bit line pattern exposed by the protection layer pattern.

The method for fabricating a DRAM cell may further include forming a first storage node pad interposed between the first storage node and the first source region of the first access transistor; forming a second storage node pad interposed between the second storage node and the second source region of the second access transistor; and forming a bit line pad interposed between the bit line pattern and the common drain region.

The forming of the insulator may include forming the second interlayer insulating layer over the first interlayer insulating layer and the protection pattern; forming an etching stopper layer over the second interlayer insulating layer; and forming a sacrificial insulating layer over the etching stopper layer.

The first interlayer insulating layer, the second interlayer insulating layer, and the sacrificial insulating layer preferably comprise silicon oxide. The protection layer and the etching stopper layer preferably comprise silicon nitride.

The forming of the first and second storage nodes may be performed by forming a conformal conductive layer over the sacrificial insulating layer and in the first and second storage node holes; forming a planarized insulator filling the first and second storage node holes over the conformal conductive layer; and blanket etching the planarized insulator and the conformal conductive layer until a top surface of the sacrificial insulating layer is exposed.

The method for fabricating a DRAM cell may include removing the exposed sacrificial insulating layer and the remainder of the planarized insulator in the first and second storage node holes, to expose outer and inner sidewalls of each of the first and second storage node, respectively; forming a dielectric layer over the etching stopper layer and the storage nodes; and forming a plate electrode layer over the dielectric layer.

To achieve these objectives and in accordance with the present invention, a DRAM cell is provided. The DRAM cell includes first and second access transistors formed over a semiconductor substrate, the first and second access transistors sharing a common drain region and having first and second source regions, respectively; first and second storage nodes electrically connected to the first and second source regions, respectively; and a protection layer pattern disposed between the common drain region and the first and second storage nodes, the protection layer pattern completely covering the common drain region.

The DRAM cell may further include a bit line pad formed over the common drain region; a first storage node pad formed over the first source region; and a second storage node pad formed over the second source region. The first and second storage nodes are electrically connected to the first and second source regions, respectively. Also, the bit line pad is electrically connected to the common drain region.

The DRAM cell may further include a bit line pattern disposed between the bit line pad and the protection layer pattern, the bit line pattern being electrically connected to the bit line pad.

The protection layer pattern may be extended to cover not only a part of the bit line pattern over the common drain region, but also a part of the first and second gate patterns of the first and second access transistors adjacent to the common drain region.

The DRAM cell may further include a first interlayer insulating layer disposed between the protection layer pattern and the first and second gate patterns of the first and second access transistors, respectively.

The protection layer pattern preferably has an etching selectivity with respect to the first interlayer insulating layer. The first interlayer insulating layer preferably comprises silicon oxide and the protection layer pattern preferably comprises silicon nitride.

As described above, in accordance with the present invention, the common drain region, the bit line pad or bit line pattern are not exposed by the storage nodes hole because of the presence of a protection layer pattern that has an etching selectivity with respect to the first and second interlayer insulating layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the invention will become apparent upon reference to the following detailed description of specific embodiments and the attached drawings, of which:

FIGS. 1 to 5 are cross-sectional views of a semiconductor substrate, at selected stages of a conventional method for fabricating a DRAM cell;

FIG. 6 schematically illustrates a layout diagram for DRAM cell in accordance with a preferred embodiment of the present invention;

FIGS. 7A to 12A are cross-sectional views of a semiconductor substrate, taken along line I-I′ of FIG. 6, at selected stages of a method for fabricating a DRAM cell according to a preferred embodiment of the present invention;

FIGS. 7B to 12B are cross-sectional views of a semiconductor substrate, taken along line II-II′ of FIG. 6, at selected stages of a method for fabricating a DRAM cell according to a preferred embodiment of the present invention; and

FIG. 13 schematically shows a DRAM cell in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more fully below with reference to the accompanying drawings, in which a preferred embodiments of the invention is shown. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate or intervening layers may also be present. Moreover, each embodiment described and illustrated below includes its complementary conductivity type embodiment as well as the conductivity type disclosed.

FIG. 6 schematically shows a part of a layout diagram for a DRAM cell according to a preferred embodiment of the present invention. Referring to FIG. 6, a pair of word line patterns, i.e. a first word line 3 a and a second word line 3 b intersect at an active region 1 defined at a predetermined portion of a semiconductor substrate, along the x-axis. Accordingly, the active region 1 is divided into three parts, a common drain region D, and first and second source regions S₁ and S₂.

The common drain region D is formed at the active region between the pair of the word line patterns. The first and second source regions S₁ and S₂ are formed at the active region outside of the common drain region D. The first word line pattern 3 a, the common drain region D and the first source region S₁ comprise a first access transistor. Likewise, the second word line pattern 3 b, the common drain region D and the second source region S₂ comprise a second access transistor.

A bit line contact hole pattern 5 is disposed over a predetermined portion of the common drain region D. A pair of bit line patterns, i.e., a first bit line pattern 7 a and a second bit line pattern 7 b intersect the pair of the word line patterns 3 a and 3 b at both sides of the active region 1, along the Y-axis. The first bit line pattern 7 a is electrically connected to the common drain region D via the bit line contact hole pattern 5. Though not shown in the current layout, the second bit line pattern 7 b is electrically connected to another common drain region D via another bit line contact hole pattern.

A protection layer pattern 8 is disposed to cover completely the common drain region D. More particularly, the protection layer pattern 8 preferably extends to cover not only a part of the first bit line pattern 7 a but also a part of the first and second word line patterns 3 a and 3 b, neighbouring the common drain region D. Preferably, the first and second source regions S₁ and S₂ are exposed by the protection layer pattern 8. This is because storage node holes are to be open to the first and second source regions S₁ and S₂. A first storage node hole pattern 9 a is disposed over the first source region S to expose the first source region S₁. Likewise a second storage hole pattern 9 b is disposed on the second source region S₂ to expose the second source region S₂.

FIGS. 7A to 12A are cross-sectional views of a semiconductor substrate, taken along line I-I′ of FIG. 6, at selected stages of a preferred method for fabricating a DRAM cell. FIGS. 7B to 12B are cross-sectional views of a semiconductor substrate, taken along line II-II′ of FIG. 6, at selected stages of the preferred method for fabricating a DRAM cell. Now, the formation of a DRAM cell by using the mask pattern schematically shown in FIG. 6 will be fully described.

Referring to FIGS. 7A and 7B, a photo mask is used to define the active region pattern 1 of FIG. 6, and a device isolation layer 53 is formed on a predetermined portion of a semiconductor substrate 51. The device isolation layer 53 may be formed by a well-known LOCOS technique or by an STI technique. A gate insulating layer 55, such as a thermal oxide layer, is then formed on the active portion of the semiconductor substrate defined by the device isolation region 53. Conductive layers to be used for a gate electrode and an insulating layer to be used for a gate capping layer are sequentially formed on the resulting structure. The conductive layer used for a gate electrode preferably comprises a polysilicon layer and polycide layer. The insulating layer used for a gate capping layer comprises a material layer that has an etching selectivity with respect to a silicon oxide layer, such as a silicon nitride layer.

Using a photo mask defining the word line patterns 3 a and 3 b of FIG. 6, the gate capping layer and the gate conductive layer are sequentially patterned to form first and second gate patterns 60 a and 60 b that intersect the active region. The first gate pattern 60 a comprises a stacked layer of a first gate electrode 57 a and a first gate capping layer pattern 59 a. Similarly, the second gate pattern 60 b comprises a stacked layer of a second gate electrode 57 b and a second gate capping layer pattern 59 b. Each gate electrode serves as a word line in the DRAM cell.

A spacer insulating layer that has an etching selectivity with respect to the silicon oxide, is then formed on the resulting structure having the gate patterns 60 a and 60 b. Preferably the spacer insulating layer comprises the same material as the gate capping layer, i.e., silicon nitride. The spacer insulating layer is then anisotropically etched to form a gate spacer 61 on sidewalls of the first and second gate patterns 60 a and 60 b.

Impurities of an opposite conductivity type to the substrate 51 are then implanted into the substrate 51 using the gate patterns 60 a and 60 b, gate spacer 61, and the device isolation region 53 as ion implantation masks, to form impurity diffusion regions. More particularly, an impurity diffusion region formed between the first and second gate patterns 60 a and 60 b, is a common drain region 63 d. An impurity diffusion region formed outside of the common drain region 63 d and neighbouring the first gate pattern 60 a is a first source region 63 a. Similarly, an impurity diffusion region formed outside of the common drain region 63 d and neighbouring the second gate pattern 60 b is a second source region 63 b. Prior to the formation of the spacer insulating layer, impurities can be implanted into the substrate using the gate patterns 60 a and 60 b and the device isolation region 53 as ion implantation masks. This implantation process is preferably performed with a low dose of about 1×10¹³ ion atoms/cm² to form a lightly doped drain (LDD) region.

Elements 63 a, 63 b, and 63 d correspond to elements S1 S2, and D in FIG. 6. Similarly, elements 57 a and 57 b correspond to elements 3 a and 3 b in FIG. 6.

The first gate electrode 57 a, the first source region 63 a, and the common drain region 63 d comprise a first access transistor. Likewise, the second gate electrode 57 b, the second source region 63 b, and the common drain region 63 d comprise a second access transistor.

Referring now to FIGS. 8A and 8B, an insulating layer 65 is formed on the resulting structure having the first and second access transistors. The insulating layer 65 preferably comprises a material that has an etching selectivity with respect to the gate capping layer 59 a and 59 b and sidewall spacer 61, such as a CVD silicon oxide. The insulating layer 65 is patterned to form holes that expose the common drain region 63 d, the first source region 63 a, and the second source region 63 b. A pad conductive layer is then preferably formed on the insulating layer 65 to fill the holes.

A planarization process is carried on the pad conductive layer until the insulating layer 65 and capping layer patterns 59 a and 59 b are exposed. This results in the formation of a bit line pad 67 d, a first storage node pad 67 a, and a second storage node pad 67 b, on the common drain region 63 d, the first source region 63 a, and the second source region 63 b, respectively. A first interlayer insulating layer 69 such as CVD oxide layer is then preferably deposited over the resulting structure having the pads.

Referring now to FIGS. 9A and 9B, the first interlayer insulating layer 69 is patterned to form a bit line contact hole that exposes the bit line pad 67 d, using a photo mask that defines the bit line contact pattern 5 shown in FIG. 6. If the process for making the bit line pad 67 d is omitted, the bit line contact hole directly exposes the common drain region 63 d. A bit line conductive layer and a bit line capping layer are sequentially formed on the first interlayer insulating layer 69 to fill the bit line contact hole. The bit line conductive layer preferably comprises a polysilicon layer and a refractory metal silicide layer. The bit line capping layer preferably comprises a material layer that has an etching selectivity with respect to a silicon oxide layer, e.g. silicon nitride.

The bit line capping layer and the bit line conductive layer are sequentially patterned using a photo mask that defines the bit line patterns 7 a and 7 b of FIG. 6, to thereby form bit line patterns, i.e., a first bit line pattern 74 a and a second bit line pattern 74 b. Accordingly, the first bit line pattern 74 a preferably comprises a stacked layer of a first bit line 71 a and a first bit line capping layer pattern 73 a. Similarly, the second bit line pattern 74 b preferably comprises a stacked layer of a second bit line 71 b and a second bit line capping layer pattern 73 b.

Elements 71 a and 71 b correspond to elements 7 a and 7 b in FIG. 6.

The fist bit line 71 a is electrically connected to the bit line pad 67 d via the corresponding bit line contact hole. Though not shown in the drawings, the second bit line 71 b is electrically connected to another bit line pad via another bit line contact hole.

A protection layer 75 is formed on the bit line patterns 74 a and 74 b and on the first interlayer insulating layer 69. The protection layer 75 preferably comprises a material that has an etching selectivity with respect to a silicon oxide layer, such as silicon nitride. A photoresist pattern 77 that defines the protection layer pattern 8 of FIG. 6 is formed on the protection layer 75. As can be seen in layout diagram of FIG. 6, the photoresist pattern 77 is formed over the common drain region 63 d. It is noted that the photoresist pattern 77 should expose first and second source regions 63 a and 63 b.

Referring now to FIGS. 10A and 10B, using the photoresist pattern 77 as an etching mask, the protection layer 75 is then patterned to form a protection layer pattern 75 b. The protection layer pattern 75 b covers the common drain region 63 d. Accordingly, the bit line pattern formed over the common drain region 63 d, i.e., the first bit line pattern 74 a, is completely covered with the protection layer pattern 75 b as shown in FIG. 10B.

The patterning of the protection layer pattern 75 b is preferably performed using an anisotropic etching process. Accordingly, the bit line patterns 74 a and 74 b exposed by the photoresist pattern 77 have sidewall spacers 75 a formed on their sidewalls, as shown in FIG. 10A. As a result, each bit line can be completely protected by either the protection layer pattern 75 b, or capping layer pattern 73 a/73 b and sidewall spacer 75 a. Furthermore, the common drain region 63 d or bit line pad 67 d electrically connected thereto are completely covered with the protection layer pattern 75 b.

Element 75 b corresponds to element 8 in FIG. 6.

An insulator 84 including a second interlayer insulating layer is formed on the resulting structure having the protection layer pattern 75 b and the sidewall spacer 75 a. More particularly, the insulator 84 preferably comprises a stacked layer of the second interlayer insulating layer 79, an etching stopper layer 81, and a sacrificial insulating layer 83. The second interlayer insulating layer 79 may comprise a planar BPSG layer.

Specifically, the BPSG layer may be deposited and an annealing process carried out to flow the deposited BPSG layer, thereby improving the step coverage. The annealing process is preferably performed at a temperature of about 850° C. to 900° C. Then a CMP process is carried out to planarize its surface. Alternatively, a CVD oxide can be deposited and then planarized by a CMP process. The etching stopper layer 81 preferably comprises a material that has an etching selectivity with respect to silicon oxide layer, e.g., silicon nitride. The sacrificial insulating layer 83 preferably comprises a CVD oxide layer.

Referring now to FIGS. 11A and 11B, the sacrificial insulating layer 83, the etching stopper layer 81, the second interlayer insulating layer 79, and the first interlayer insulating layer 69 are sequentially etched to form first and second storage node holes 85 a and 85 b, using a photo mask that defines first and second storage node hole patterns 9 a and 9 b of FIG. 6. The first and second storage node holes 85 a and 85 b respectively expose the first storage node pad 63 a and the second storage node pad 63 b. Since the protection layer pattern 75 b and the sidewall spacer 75 a each have an etching selectivity with respect to the first and second interlayer insulating layers 69 and 79, the exposure of the bit lines 71 a and 71 b by the storage node holes 85 a and 85 b can be avoided. In particular, since the protection layer pattern 75 b covers the common drain region 63 d and the bit line pad 67 d, parts of the first interlayer insulating layer 69 formed over the common drain region 63 d and the bit line pad 67 d, but underlying the protection layer pattern 75 b are not etched. Accordingly, even in the case of misalignment during the photo process for defining the first and second storage node holes 85 a and 85 b, the exposure of the common drain region 63 d and the bit line pad 67 d can be prevented.

If the step of forming the first and second storage node pads 67 a and 67 b is not implemented, the first and second storage node holes 85 a and 85 b directly expose the first and second source regions 63 a and 63 b respectively. At this time, the first and second gate electrodes 57 a and 57 b can be protected by the gate capping layer patterns 59 a and 59 b and the gate spacer 61.

Elements 85 a and 85 b correspond to elements 9 a and 9 b in FIG. 6.

A conformal conductive layer 87 for a storage node is then deposited on the sacrificial insulating layer 83 and in the storage node holes 85 a and 85 b. The storage node conductive layer 87 may be made, for example, of doped polysilicon.

A planarized insulating layer is then formed on the storage node conductive layer 87 to fill the storage node holes 85 a and 85 b. The planarized insulating layer may comprise a photoresist layer or an oxide layer that has a good step coverage, such as a CVD oxide layer. A blanket etching process is then carried out on the planarized insulating layer and the storage node conductive layer until a top surface of the sacrificial insulating layer 83 is exposed, thus forming first and second storage nodes 87 a and 87 b in the first and second storage node holes 85 a and 85 b (refer to FIG. 12).

The exposed sacrificial insulating layer 83 and the remainder of the planarized insulating layer within the first and second storage nodes 87 a and 87 b are removed to expose inner and outer walls of the storage nodes 87 a and 87 b. At this time, the second interlayer insulating layer 79 is not etched because of the presence of the etching stopper layer 81. After that, a dielectric layer 89 and a plate electrode 91 are sequentially formed on the second interlayer insulating layer 79 and on the storage nodes 87 a and 87 b, as shown in FIGS. 12A and 12B.

FIG. 13 is a schematic cross-sectional view of a DRAM cell that is fabricated by above-mentioned method, taken along line II-II′ of FIG. 6. Referring to FIG. 13, an active region is defined on a predetermined portion of a semiconductor substrate 51 by a device isolation layer 53. Adjacent first and second gate patterns 60 a and 60 b are disposed on the active region. A common drain region 63 d is formed at the active region between the adjacent gate patterns 60 a and 60 b. Source regions 63 a and 63 b are defined on the active regions outside of the common drain region 60 d. More particularly, a first source region 63 a is formed at the active region outside of the first gate pattern 60 a, and a second source region 63 b is formed at the active region outside of the second gate pattern 60 b.

The common drain region 63 d, the first source region 63 a, and the first gate pattern 60 a together comprise a first access transistor. Similarly, the common drain region 63 d, the second source region 63 b, and the second gate pattern 60 b together comprise a second access transistor. Each gate pattern 60 a and 60 b is electrically insulated to the underlying substrate 51 by a gate oxide 55. Also, each gate pattern 60 a and 60 b comprises a stacked layer of a gate electrode and a gate capping layer. The gate capping layer preferably comprises a material that has an etching selectivity with respect to silicon oxide, such as silicon nitride. Furthermore, each of the gate patterns 60 a and 60 b further comprises a gate spacer 61 on its sidewalls. Preferably, the gate spacer 61 comprises the same material as the gate capping layer, i.e., silicon nitride. Accordingly, the gate electrode is covered with the gate capping layer and gate spacer.

An insulating layer 65, having optional conductive pads 67 a, 67 b, and 67 d, is disposed on the device isolation region 53. More particularly, a bit line pad 67 d is disposed on the common drain region 63 d between adjacent access transistors, a first storage node pad 67 a is disposed on the first source region 63 a, and a second storage node pad 67 b is disposed on the second source region 63 b. First and second interlayer insulating layers 69 and 79 and an etching stopper layer 81 are sequentially stacked on the insulating layer 65.

The first and second interlayer insulating layers 69 and 79 and an etching stopper layer 81 have first and second storage node holes exposing the first and second storage node pads 67 a and 67 b, respectively. In the absence of the optional first and second storage node pads 67 a and 67 b, the first and second storage node holes penetrate to the insulating layer 65 and extend downward to the first and second source regions 63 a and 63 b respectively.

First and second storage nodes 87 a and 87 b are disposed in the first and second storage node holes respectively. The first and second storage nodes 87 a and 87 b are electrically connected to the first and second storage node pads 67 a and 67 b, respectively.

The first and second storage nodes can be of a cylindrical type, as shown in FIG. 13. The first and second storage nodes 87 a and 87 b preferably are higher than the surface of the etching stopper layer 81. In the absence of the optional first and second storage node pads 67 a and 67 b, the first and second storage nodes 87 a and 87 b directly contact with the first and second source regions 63 a and 63 b, respectively.

A protection layer pattern 75 b is disposed between the first and second interlayer insulating layers 69 and 79 and is aligned over the common drain region 63 d. The protection layer pattern 75 b preferably comprises a material that has an etching selectivity with respect to the first and second interlayer insulating layer 69 and 79, such as silicon nitride. Accordingly, the protection layer pattern 75 b can be disposed between the common drain region 63 d and the storage nodes. As a result of this, the common drain region 63 d can be protected by the protection layer pattern 75 b even if there is a misalignment during the photo process for defining the first and second storage node holes.

A bit line pattern 74 a is disposed under the protection layer pattern 75 b and on the first insulating layer 69 to be electrically connected to the bit line pad 67 d via a bit line contact hole in the first interlayer insulating layer 69. In the absence of the optional bit line pad 67 d, the bit line pattern 74 a is directly connected to the common drain region 63 d. The bit line pattern 74 a preferably comprises a stacked layer of a bit line 71 a and a bit line capping layer 73 a. The bit line 71 a comprises a refractory metal silicide. The bit line capping layer 73 a comprises a material that has an etching selectivity with respect to silicon oxide, such as silicon nitride.

As described above, the DRAM cell of the current invention includes the protection layer pattern 75 b over the common drain region 63 d. Furthermore, the protection layer pattern 75 b completely covers the bit line pattern 74 a, which is electrically connected to the common drain region 63 d. Accordingly, the respective storage nodes 87 a and 87 b and the common drain region 63 d can be advantageously separated both spatially and electrically.

The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiment set forth above. Rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. For example, the first and second storage nodes can be a box-type nodes completely filling the node holes. In addition, an HSG silicon layer may also be formed on the surfaces of the storage nodes. 

What is claimed is:
 1. A method for fabricating a DRAM cell, comprising: forming first and second access transistors sharing a common drain region over a semiconductor substrate; forming a first interlayer insulating layer on the semiconductor substrate and the first and second access transistors; providing a protection layer pattern completely covering the common drain region on the first interlayer insulating layer; forming an insulator including a second interlayer insulating layer over the first interlayer insulating layer and the protection layer pattern; patterning the insulator and the first interlayer insulating layer to form first and second storage node holes, respectively exposing a first source region of the first access transistor and a second source region of the second access transistor; and forming first and second storage nodes in the first and second storage node holes, respectively.
 2. A method for fabricating a DRAM cell, as recited in claim 1, wherein the forming of the first and the second access transistors comprises: forming a device isolation layer over a selected portion of the semiconductor substrate to define a primary active region; forming first and second insulated gate patterns crossing over the primary active region to divide the primary active region into first through third active regions; forming sidewall spacers on sidewalls of the first and second insulated gate patterns; and forming the common drain region at the first active region between the first and second insulated gate patterns, and the first and the second source regions at the second and third active regions next to the first and second insulated gate patterns, respectively.
 3. A method for fabricating a DRAM cell, as recited in claim 1, further comprising a step of forming a bit line pattern between the protection layer pattern and the common drain region.
 4. A method for fabricating a DRAM cell, as recited in claim 3, wherein the forming of the protection layer pattern comprises: forming the protection layer over the first interlayer insulating layer and over the bit line pattern; forming a photoresist pattern over the protection layer to cover the common drain region and expose the first and second source regions; and anisotropically etching the protection layer using the photoresist pattern as an etching mask to form the protection layer pattern completely covering the bit line pattern over the common drain region, while concurrently forming sidewall spacers on sidewalls of the bit line pattern exposed by the protection layer pattern.
 5. A method for fabricating a DRAM cell, as recited in claim 3, further comprising: forming a first storage node pad interposed between the first storage node and the first source region of the first access transistor; forming a second storage node pad interposed between the second storage node and the second source region of the second access transistor; and forming a bit line pad interposed between the bit line pattern and the common drain region.
 6. A method for fabricating a DRAM cell, as recited in claim 1, wherein the forming of the insulator comprises: forming the second interlayer insulating layer over the first interlayer insulating layer and the protection layer pattern; forming an etching stopper layer over the second interlayer insulating layer; and forming a sacrificial insulating layer over the etching stopper layer.
 7. A method for fabricating a DRAM cell, as recited in claim 6, wherein the first interlayer insulating layer, the second interlayer insulating layer, and the sacrificial insulating layer comprise silicon oxide, and wherein the protection layer and the etching stopper layer comprise silicon nitride.
 8. A method for fabricating a DRAM cell, as recited in claim 6, wherein the forming of the first and second storage nodes comprises: forming a conformal conductive layer over the sacrificial insulating layer and in the first and second storage node holes; forming a planarized insulator filling the first and second storage node holes over the conformal conductive layer; and blanket etching the planarized insulator and the conformal conductive layer until a top surface of the sacrificial insulating layer is exposed.
 9. A method for fabricating a DRAM cell, as recited in claim 8, further comprising: removing the exposed sacrificial insulating layer and the remainder of the planarized insulator in the first and second storage node holes, to expose outer and inner sidewalls of each of the first and second storage nodes, respectively; forming a dielectric layer over the etching stopper layer and the first and second storage nodes; and forming a plate electrode layer over the dielectric layer. 